The distribution of 3H-melatonin in chick embryos and mice

Tritium-labelled melatonin was injected into adult mice and embryonic chicks, and various tissues were assayed for radioactivity over a period of time. Of the chick somatic tissues, only the pineal gland concentrated radioactivity significantly. There were no significant changes in tissue concentration of radioactivity over a 12 h. sample period. The physiological significance of the concentration of melatonin by the embryonic chick pineal gland has not been definitely established. Endocrine tissues of the mouse, i.e. thyroid gland, adrenal gland, and ovary, and also skin, concentrated radioactivity significantly more than did other body tissues over a 24 h. sample period, indicating that these might be target organs of the hormone. These tissues also showed indications, not exhibited by the other tissues, of following a circadian rhythm in their ability to concentrate melatonin.